A thermal power plant and a nuclear power plant generally provided with a system which drives a turbine with steam generated by a steam generator and returns condensate water to the steam generator. Since, pipes and/or devices of such system may be damaged due to corrosion during operation, countermeasures to such damage have been taken for reducing the corrosion.
For example, in secondary systems of current pressurized water nuclear power plants, such countermeasures as makeup water management and management of water treatment chemicals are being taken for preventing infiltration of impurities into the system in order to prevent corrosion troubles in steam generators and turbines. In order to suppress corrosion of devices and pipes that constitute a system, countermeasures are taken to obtain a deoxidized and reducing atmosphere by pH control with use of pH adjusters and injection of hydrazine. Furthermore, various other countermeasures or procedures have been taken, such as installation of a desalting device and the proper operation thereof for removing infiltrated impurities out of the system, installation of a clean-up system and a steam generator blowdown collection system, and installation of a deaerator for reducing dissolved oxygen.
The deaerator is placed to deaerate circulating water of the system and to reduce oxygen from transferring to the steam generator. The deaerator acts to suppress increase in corrosion potential of structural members due to oxygen contribution. As oxygen concentration increases, cracking such as intergranular corrosion cracking and stress corrosion cracking occurs due to the potential increase.
Meanwhile, elution of metal ions from pipes and the like is a typical phenomenon that occurs in high temperature hot water. Elution of metal ions causes operational problems attributed to corrosion of structural members as well as pipes and other members, and exerts various influences such as increase in frequency of maintenance. Moreover, eluted metal ions are deposited and crystallized as an oxide at high temperature portions in the system, such as pipe surfaces and the steam generator, which causes a phenomenon of corrosion cracking due to potential increase. Since the adhering oxide causes deterioration in heat transfer, the oxide needs to be removed on a periodic basis by chemical cleaning.
Thus, such phenomena as metal elution and corrosion may gradually be accumulated during a long-term plant operation or running and may possibly cause disaster at some point without notice. In order to obviate such phenomena, chemicals such as ammonia and hydrazine are injected for pH control to implement deaeration so as to reduce iron elution from the system as a countermeasure to prevent inflow of iron into the steam generator.
In order to eliminate alkali concentration in a clevis portion, various suggestions have been made for water quality control, such as chloride ion concentration management and dissolved oxygen concentration control.